Logical reasoning PrepTest 140 · Section 2 · Question 25

Incorrect. The entire reasoning described is predicated on the stars revolving around Earth, so it doesn't assume anything about if that weren't the case.

Incorrect. The argument only requires that the stars don't move at "tremendously great" speeds—they can vary in their speeds as long as they're all slower than that.

Incorrect. This answer represents the opposite of the old astronomers' viewpoint, so it's not a necessary premise.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Assumption Ð The stars revolve around Earth
Implication/Conclusion Ð The stars must be within a few million miles of Earth
Reasoning Ð If they were farther away, they'd have to be moving at "tremendously great speeds"

Answer Anticipation:
This (ridiculous . . . ) argument has an interesting structure. It reaches its conclusion that the stars are only a few million miles from Earth by showing an implication of that not being true. Clearly, these Astronomers believed that the implication was ridiculous and thus couldn't be true (unlike the rest of their argument . . . ).

However, the argument never actually states that the implication—that the stars would need to be moving at great speeds if they were farther away—is ridiculous, nor does it present evidence that it's false.

So the argument relies on it being impossible for stars to move at great speeds.

Answer Explanation:
This answer establishes that stars can't move at great speeds, which means that (under the assumption the stars revolve around the Earth) they can't be farther away than a few million miles. If the stars can move at such speeds, then they could be farther away, undermining the argument.

Key Takeaway:
This entire argument is pretty ridiculous. Why did the LSAT include it? To try to throw you off with that ridiculousness. In assuming that a ridiculous statement is false, the LSAT is trying to get you to miss that it's still an assumption to rule out a ridiculous possibility without proof.

Incorrect. This is a very tempting answer choice. However, the argument in the stimulus says that the stars can't be more than a few million miles from Earth—that could mean that they're all within a million miles from Earth. Since the stimulus includes a possibility where this answer choice doesn't apply, it can't be necessary for the argument to work.